Riprap-sill shoreline at NCCOS lab in North Carolina. Credit: NOAA.

In many places, estuarine shorelines are protected from erosion by riprap—a jumble of rock and other hard materials piled along the shore. More recently, living shorelines have become the focus of study as an alternative approach that preserves ecological function as well as providing shoreline protection. For example, a riprap-sill structure is a type of living shoreline that combines a rock sill placed low in the intertidal zone with native vegetation planted between the sill and the shore.

NCCOS has built such a living shoreline at their lab on Pivers Island, North Carolina, and shown that marshes with sills support equal or greater numbers of fish than vegetated shorelines. However, little research has been conducted to compare how a riprap-sill habitat is used by estuarine animals compared to a riprap-only installation.

A recent NCCOS-sponsored publication compared the density and diversity of fish and blue crabs via weekly sampling along a riprap-sill shoreline, a riprap shoreline, and a shoreline fringed with smooth cordgrass (Spartina alterniflora) marsh in the Delaware Coastal Bays. In measuring overall fish density and diversity, riprap-sill was more similar to the smooth cordgrass shoreline than to the traditional riprap shoreline. In addition, several species (the Atlantic silverside, silver perch, and bay anchovy) had significantly greater densities at the riprap-sill shoreline than along riprap, and these densities did not differ from the smooth cordgrass habitat. No species had significantly greater densities along the riprap than along either the smooth cordgrass or the riprap-sill shoreline.

Traditional riprap. Credit: NOAA.

These results provide evidence for the biological advantage of riprap-sill over traditional riprap as a shoreline modification structure; confirmation by further studies at different locations is warranted.